Restored tooth failures attributed to tooth fracture are one of the primary obstacles to life long oral health. And despite progressive improvements in dental instruments and restorative materials, tooth fractures have remained as a prevalent clinical issue. Recent studies have shown that fatigue cracks are often identified in the dentin of restored teeth and appear to originate from the cavosurface margin. Based on these observations, restored tooth fracture is believed to occur as a consequence of fatigue. Microcracks in the dentin coalesce into a single well-defined crack under cyclic masticatory loads and crack propagation continues until a critical length facilitates tooth fracture. Both the rate of crack initiation in dentin, and the corresponding likelihood of restored tooth failure, are undoubtedly dependent on the fatigue strength of dentin. However, there is currently little understanding of the fatigue strength of dentin and the changes that occur with aging and other processes. The general hypothesis of the proposed research is that restored tooth fracture is attributed to changes in the fatigue properties of dentin that occur with physiological processes, aging, and demineralization. Prior to determining the influence of these factors on mechanical properties, a fundamental understanding of the fatigue strength of dentin is needed. The specific hypothesis to be tested is that human dentin exhibits an "endurance strength" which is a magnitude of cyclic stress below which fatigue failure of the tissue will not occur. In this study the fatigue strength of bovine and human dentin will be determined using flexural fatigue loading. The proposed investigation will contribute to our understanding of the structure-property relationships in dentin, and provide a foundation of knowledge for future studies regarding the influence of physical and chemical changes on the fatigue properties of human teeth. Therefore, results from this study could have tremendous impact on the clinical techniques and procedures used for restorative dentistry.